1. Field of the Invention
This invention relates to tracking of assets and, more particularly, to an asymmetrical high frequency (HF) messaging system used in the tracking of assets, including goods and vehicles.
2. Description of the Prior Art
U.S. Pat. No. 5,588,005 to Ali et al. describes the tracking of assets, including goods and vehicles, using the Global Positioning System (GPS). While goods are an example of assets that need to be tracked, the containers, container trucks and railcars in which the goods are shipped are themselves assets which need to be tracked, not just because of the goods they carry, but also because they represent capital assets typically of a leasing company not associated with the carrier.
The mobile tracking unit used in the Ali et al. system includes a navigation set, such as a Global Positioning System (GPS) receiver or other suitable navigation set, responsive to navigation signals transmitted by a set of navigation stations which can be either space- or earth-based. In each case, the navigation set is capable of providing data indicative of the vehicle location based on the navigation signals. In addition, the mobile tracking unit can include a suitable electromagnetic emitter for transmitting to a remote location the vehicle's position data and other data acquired with sensing elements in the vehicle.
There are two modes of communication for the asset tracking units. A first of these modes ("central station" communication) is that in which the communication is carried out between a central manager or station and the individual tracking units. This communication usually takes place through a satellite link. The second mode ("mutter mode" communication) is the local area network, referred to in the Ali et al. patent as the "mutter" mode, in which a subset of tracking units communicate with each other in a mobile dynamically configured local area network (LAN).
The first of these modes is the primary communication link for tracking the assets. Mutter mode communication is used as a secondary communication mechanism to conserve power. Ali et al. specify a protocol for mutter mode communication in their patent. The prime requirement of any protocol is that it be simple for implementation purposes and at the same time be robust under different failure modes. The protocol developed for the mutter mode makes use of the fact that there exists a two-way communication channel between the tracking units and the central station. Since the central station has use of a fairly powerful computer, the central station's processing power is used in setting up and maintaining the mutter mode network. This enables keeping the mutter mode protocol simple and reduces the complexity at individual tracking units which may number in the hundreds of thousands. In conjunction with the protocol for the central station communication, the protocol for mutter mode communication is very similar. The frame structure developed for the central station communication protocol can be used for the mutter mode communication as well. This further simplifies implementation of the mutter mode communication.
One of the options available for tracking railcars involves transmission of a high frequency signal from the tracked railcar. The high frequency (HF) spectrum, roughly 3 to 25 MHZ, may be used to successfully transmit a slow speed inbound (from the asset to a base) digital data stream. One available system depends upon there being an outbound (from the base to the asset) link for control. This complementary link infrastructure may be costly to establish and maintain. It would therefore be desirable to devise an asymmetrical system which uses an inbound link only with the possible addition of information provided to the asset by services such as the Global Positioning System (GPS) service.